Void Space versus Surface Functionalization for Proton Conduction in Metal–Organic Frameworks

Abstract

Void space and functionality of the pore surface are important structural factors for proton-conductive metalorganic frameworks (MOFs) impregnated with conducting media. However, no clear study has compared their priority factors, which need to be considered when designing protonconductive MOFs. Herein, we demonstrate the effects of void space and pore-surface modification on proton conduction in MOFs through the surface-modified isoreticular MOF-74(Ni) series [Ni 2 (dobdc or dobpdc), dobdc = 2,5-dihydroxy-1,4-benzenedicarboxylate and dobpdc = 4,4'-dihydroxy-(1,1'biphenyl)-3,3'-dicarboxylate]. The MOF with lower porosity with the same surface functionality showed higher proton conductivity than that with higher porosity despite including a smaller amount of conducting medium. Density functional theory calculations suggest that strong hydrogen bonding between molecules of the conducting medium at high porosity is inefficient in inducing high proton conductivity.